1. Technical Field
The present invention relates generally to wide-band code division multiple access (WCDMA) receivers and other receivers for wireless communications. More particularly, the present invention relates to system and method for providing an automatic gain control (AGC) system with high dynamic range (over 100 dB), whereby the AGC can operate over a broad range of varying signal level while maintaining a good signal-to-noise (SN) ratio performance over the entire signal varying range and maintaining inter-modulation components at low levels.
2. Description of Related Art
In general, wireless communication systems employing wide-band receivers (e.g., WCDMA receivers) typically share one carrier channel among a plurality of users. As a result, the signal strength may vary over a very wide range depending on factors such as the distance between a mobile station and a service station and the number of neighboring subscribers sharing the same RF carrier. Indeed, the signal received by a service center or by a mobile station may vary over a 90 dB range and a received signal may even be as high as -25 dBm.
A conventional wide-band receiver typically comprises a RF-IF (radio frequency--intermediate frequency) section that inlcudes an AGC circuit. In general, the function of the AGC circuit is to process an input IF signal to generate an IF signal with a fixed power level at the output of the AGC (regardless of the level of the input IF signal level). This fixed signal level is required for further IF processing, both within the RF-IF section and in a base-band processing unit. A conventional AGC circuit is typically designed as a closed loop AGC circuit comprising a gain variable amplifier (GVA) and some control hardware/software to control the gain of the GVA.
FIG. 1 is a block diagram of a conventional receiver comprising a single, closed loop AGC circuit according to the prior art. The receiver comprises an antenna 100 for receiving a RF signal. A LNA (low noise amplifier) 101 amplifies the RF signal and the amplified RF signal is filtered by a band-pass filter 102 to generate a RF signal (e.g., 2120 MHz to 2180 MHz). A down-converter 103 processes the bandpass-filtered RF signal to generate an IF signal. The IF signal comprises a plurality of channels (e.g., 12 channels with a channel bandwidth of 5 MHz in the range 2120 MHz to 2180 MHz). A SAW (surface acoustic wave) filter 104 filters the IF signal to select a desired channel within the IF signal. Each of these components and their respective functions are well-known the art.
The IF signal (i.e., selected channel) is fed to a closed loop AGC circuit 105 comprising a GVA 106 and an IF sensor and control signal generator 107. The GVA 106 amplifies the IF signal to predetermined power level. The IF signal is further processed by other IF processing units 108 (e.g. filters, amplifiers, etc). A power splitter 109 divides the IF power received from the IF processing units 108. A demodulator 110 receives an IF signal from one output from the power splitter 109 and extracts a baseband signal from the IF signal. Another output of the power splitter 109 is connected to the IF sensor and signal generator 107 to provide a feedback control of the gain of the GVA 106. In particular, the IF sensor and signal generator 107 comprises a logarithmic amplifier which converts the IF Sample signal to a voltage signal (based on the signal level of the IF Sample). The voltage level is compared to a Desired level setting signal to generate an appropriate control signal for the GVA 106 based on the comparison. The gain of the GVA 106 will be automatically adjusted to reach a balance under which the output signal of the GVA 106 will be fixed at the predetermined level.
One problem associated with the conventional AGC architecture based on the single, closed-loop design is that it does not provide high dynamic range with respect to the range of RF power levels that may be input to the receiver. A detailed analysis of the system of FIG. 1 with respect to dynamic range will now be provided. For purposes of illustration, it is assumed that the system of FIG. 1 is a wide-band receiver that operates in a frequency range of 2120 MHz to 2180 MHz. It is further assumed that the channel bandwidth is 5 MHz per channel and that the RF signal level input to the receiver varies between -25 dBm/channel and -115 dBm/channel. In addition, the following system requirements are assumed. First, the required signal-to-noise (S/N) ratio of the demodulated baseband signal should not be less than 0 dB (the baseband processing unit of the receiver has a processing gain, so the demodulated baseband data is not required to have a high margin of S/N ratio). Second, the signal level at the input of any device in the receiver chain should not be higher than -18 dBc of the IIP.sub.3 value of that device (where the IIP.sub.3 parameter denotes the input third inter-modulation cross-point in dBm) so as to ensure that the IM.sub.3 of the receiver chain will be at least -36 dBc (where the IM.sub.3 parameter is the third inter-modulation component in dBc). It is to be understood that the above system requirements are typical for WCDMA systems or CDMA- 2000 systems.
In addition, for illustrative purposes, the GVA 106 is assumed to have the performance parameters as set forth in Table 1 below. It is to be understood that these performance parameters correspond to parameters of a GVA of current technology such as the RF2607 GVA by RF Micro Devices. Another GVA device that may be considered is the Q5500 by QUALCOMM, which has similar performance as that of RF2607. As shown in Table 1, the noise figure (NF) and IIP.sub.3 parameters of the illustrative GVA (e.g., RF2607) vary with the change in gain of the GVA.
TABLE 1 Gain 45 30 20 10 0 -10 -20 -30 -45 (dB) NF (dB) 5 6 10 18 25 34 42 48 60 IIP.sub.3 -46 -34 -23 -18 -14 -12 -7 -4 -3 (dBm)
In addition, for illustrative purposes, the signal level (SL), noise figure (NF), GVA gain (GA), and S/N ratio S/N at different locations in the receiver chain of FIG. 1 using a single, closed loop AGC topology are listed below in Table 2.
TABLE 2 SL at the input of -115 -105 -95 -85 -75 -65 -55 -45 -35 -25 the receiver (dBm) SL at the input of -111 -101 -91 -81 -71 -61 -51 -41 -31 -21 the GVA (dBm) GA of the GVA 45 35 25 15 5 -5 -15 -25 -35 -45 (dB) SL at the output of -66 -66 -66 -66 -66 -66 -66 -66 -66 -66 the GVA (dBm) NF of the GVA 5.0 5.5 8.0 14 21 29 38 45 52 60 (dB) NF at the output 6.9 7.0 7.9 11.3 17.3 25.1 34.0 41.0 48.0 56.0 of the GVA (dB) Noise floor at the -100 -100 -99.1 -95.7 -89.7 -81.9 -73.0 -66.0 -59.0 -55.0 output of the GVA (dBm) S/N at the exit of 34 34 33 30 24 16 7 0 -7 -11 the GVA (dB)
The performance of the conventional single, closed loop AGC with respect to dynamic range will now be discussed in to detail with respect to the illustrative system parameters and values set forth in Tables 1 and 2 above. As noted above, the SL at the input of the receiver (i.e., antenna 100) is assumed to vary in the range from a low level of -115 dBm/channel to a high level of -25 dBm/channel (as shown in Table 2 varying in the range in increments of 10 dBm). The SL at the output of the GVA 106 is maintained at a fixed level in accordance with the closed loop requirement. As shown in Table 2, the fixed level at the output of the GVA 106 is assumed to be -66 dBm. This fixed level is based on factors such as (1) the assumed maximum available gain GA (e.g., 45 dBm) of the GVA for the lowest SL at the input of the receiver (e.g., -115 dBm) and (2) an assumed total gain of 4 dBm for the "receiver chain" up to the GVA 106 (as indicated in Table 2 by the difference between the SL at the input of the GVA and the SL at the input of the receiver). It is to be understood that the "receiver chain" up to the GVA 106 comprises the LNA 101, the BPF 102, the down-converter 103, and the IF SAW filter 104 as shown in FIG. 1.
As shown in Table 1, the IIP.sub.3 of the GVA 106 is assumed to be -3 dBm when the gain of the GVA 106 is -45 dB (which is the gain of the VGA for the highest input signal level of -25 dBm as shown in Table 2 below) and the signal level at the input of the VGA 106 can not be higher than -21 dBm to satisfy the system IM.sub.3 requirement. In particular, the following requirement should be satisfied:
.vertline.IM.sub.3.vertline.=2*(IIP.sub.3 -(GA+SL.sub.max)).gtoreq.36(dBc) Eqn. 1
where IM.sub.3 is the third inter-modulation suppression in dB at the exit of the GVA and IIP.sub.3 is the input third inter-modulation cross-point of the GVA in dBm (as noted above), where GA is the chained receiver gain up to the input of the GVA 106 in dB, and SL.sub.max is the maximum acceptable RF signal power level in dBm at the input of the receiver. Therefore, based on the assumed system requirements, the total chained gain of the receiver up to the GVA 106 can not be higher than 4 dB. It is further assumed that the chained noise figure of the receiver up to the input of the GVA 106 is 6 dB, a fare accessible value.
As indicated in Table 2, for a weak signal (-115 dBm/channel) the gain of the GVA 106 is set to its maximum +45 dB and its NF is 5 dB. The (chained) NF at the output of the GVA can be calculated using the following formula: ##EQU1##
where NF denotes the chained noise figure at the output of a current component in db, F2 denotes the noise factor (linear) of the current component, F1 denotes the noise factor (linear) of a previous component (or the chained noise factor (linear) of all previous components), and GAN1 denotes the gain (linear (i.e., not dB)) of the previous component (or chained gain of all previous components) Therefore, using Eqn. 2, the NF at the output of the GVA for, the SL of-115 dBm/channel is equal to 10 log(3.98+(3.16-1)/2.51)=6.85 (dB). In addition, the corresponding noise floor at the output of the GVA is can be calculated using the following formula: EQU NOISEFLOOR=-174(dBm/Hz)+10 log(BW)+NF Eqn. 3
where -174(dBm/Hz) is the noise power density for a device at room temperature, BW is the channel bandwidth, and NF denotes the noise figure at the output of the current device (as calculated via Eqn. 2). Thus, the noise floor at the output of the GVA corresponding to the SL of-115 dBm/channel is equal to -174 (dBm/H.sub.z)+10 log(5 MHz)+6.85=100.1 dBm/5 MH.sub.z. The SL at the output of the GVA is -111+45=-66(dBm/5 MH.sub.z). The S/N ratio for the weak signal is calculated as the difference between the SL at the exit of the GVA and the noise floor at the output of the GV, i.e., -66 dBm--100 dBm=+34 dB, which is very good.
On the other hand, for the strongest signal (-25 dBm/5 MH.sub.z), the GVA gain is set to -45 dB to keep its output at the fixed level of -66 dBm. The NF at the output of the GVA is 10 log (3.98+(10.sup.6 -1)/2.51)=56.0 dB, the noise floor at the output of the GVA is -55.0(dBm/5 MH.sub.z), and the S/N ratio at the output of the GVA is -11 dB. This S/N ratio does not satisfy the system requirement.
Indeed, as is evident from Table 2, the dynamic range of the receiver using the conventional single, closed loop AGC 105 based on the assumed system parameters ranges from -45 dBm to -115 dBm, i.e., a dynamic range of 70 dB, since only within this range the S/N ratio at the exit of the GVA is 0 dB or better and IM.sub.3 requirement is satisfied. The S/N performance of the conventional closed loop AGC 105 of FIG. 1 is poor when the input signal is strong. This is due to the GVA 106 having to operate in a high attenuation state for strong signals where its noise figure is non-linearly increased to a very high value. The dynamic range of the AGC 105 and, consequently, the receiver is thus limited when the GVA is forced to operate in such as high noise attenuation state.
Furthermore, the dynamic range of a receiver using the conventional single, closed loop AGC 105 of FIG. 1 is limited by the IIP.sub.3 parameter of the GVA 106 employed in the AGC 105. Consequently, because the IM.sub.3 system requirement (Eqn. 1) should be satisfied, either the chained receiver gain up to the input of the GVA (i.e., GA of Eqn. 1) or the maximum acceptable signal level (i.e., SL.sub.max of Eqn. 1) has to be limited.
Accordingly, there is a need for an improved AGC system that can overcome the above-mentioned problems and efficiently operate over a broad range of input signal levels. In particular, an AGC system that can provide high dynamic range of operation and improved S/N noise ratio performance, while keeping the inter-modulation components suppressed, is highly desirable.